Photoiniferter technology is a recently developed technology which allows for the polymerization of block copolymers from various monomers An advantage of photoiniferter technology is that it allows for the "tailoring" of block copolymers made therewith. The control of the polymerization provided by the photoiniferter technology permits "tailoring" of the block copolymers formed thereby so that a polymer having a wide spectrum of physical properties can be prepared. Such high precision tailoring is not possible with previously known polymerization methods such as thermal polymerization.
The term "iniferter", or "photoiniferter" as it is also known, refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent, and terminator, the term "iniferter" being a word formed by the underlined portions of the terms identifying these functions. The photo portion of the term indicates that the polymerization is photolytically induced. This term and its use in the production of block copolymers is well known, particularly because of the work of Takayuki Otsu of the Department of Applied Chemistry, Osaka City University, Osaka, Japan. This work is discussed, for example, in an article by Otsu et al entitled "Living Radical Polymerizations in Homogeneous Solution by Using Organic Sulfides as Photoiniferters", Polymer Bulletin, 7, 45-50 (1982), an article by Otsu et al entitled "Living Mono-and Biradical Polymerizations in Homogeneous System Synthesis of AB and ABA Type Block Copolymers", Polymer Bulletin, 11, 135-142 (1984), and in European Patent Application No. 88303058.7, Publication No. 0 286 376, publication date Oct. 12, 1988.
Copending U.S. application Ser. No 07,356,650, filed May 19, 1989, which is a Continuation-In-Part of U.S. application Ser. No 7/212,594, Ali, et al., filed June 28, 1988 (assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers having the requisite physical properties making them suitable for use in pressure-sensitive adhesive compositions. The control of the polymerization permits tailoring of the reinforced acrylic block copolymer to provide a balance of adhesion, cohesion, stretchiness and elasticity to make a successful pressure-sensitive adhesive.
Copending U.S. application Ser. No. 07/212,593, filed June 28, 1988, Andrus Jr. et al., (also assigned to the assignee of the present case) discloses the use of iniferter technology in the preparation of acrylic block copolymers which can be tailored to provide optical clarity and resistance to oxidative and photochemical degradation which are employed to make shaped articles, sheet materials, and the like.
Copending U.S. application Ser. No. 07/393,550, filed Aug. 14, 1989, Kumar, et al., provides novel siloxane iniferter compounds which can be used in making tailor-made vinyl-siloxane block copolymers. The control of the polymerization provided by the novel siloxane iniferter compounds permits "tailoring" of the vinyl-siloxane block copolymers so that a wide spectrum of physical properties can be introduced.
Copending U.S. application Ser. No. 7/393,557, filed Aug. 14, 1989, Kumar, et al., discloses flexible substrates coated with a release coating comprising the vinyl-siloxane block copolymers prepared according to copending U.S. application Ser. No. 07/393,550.
While photoiniferter technology provides a unique way to make precisely tailored block copolymers which cannot be made by conventional techniques, such photopolymerization reactions can sometimes take an extended period of time to occur. Reaction times on the order of 2 to 50 hours, depending upon the components used, with the majority of reactions requiring 24 hours or more have been found to be typical, depending upon the intensity of the radiation with faster reaction times being observed at greater intensities. None of the above references disclose or suggest any methods of accelerating the photoiniferter polymerization processes.
Okuzuwa, Hirai and Hakishima, Journal of Polymer Science, Volume 7, 1039-1053, 1969, discusses the thermal polymerization of acrylates and methacrylates with AIBN (azo-isobutyronitrile) in the presence of ZnCl.sub.2 and SnCl.sub.4. The thermal polymerizations of Okuzuwa are conducted in bulk since, according to Okuzuwa, ZnCl.sub.2 acts as a complexing agent with methyl methacrylate (MMA) resulting in a decrease in activation energy. No solvent was found which dissolved the complex without causing its dissociation. Thus, no polymerization rate acceleration was observed in the presence of solvents.
Levens U.S. Pat. No. 4,421,822 (assigned to the assignee of the present case) teaches that the conventional photopolymerization of acrylates in solution or in film can be carried out in the presence of oxygen wherein oxidizable tin salts are added. However, rate enhancement in the absence of oxygen is not discussed. Moreover, no data is provided on the extent of rate enhancement in the presence and absence of oxidizable tin salts. Furthermore, Levens does not teach or suggest the use of such oxidizable tin salts as accelerators in photoiniferter polymerizations.
A need therefore exists for an accelerated photoiniferter polymerization technique which can substantially increase the reaction rate of photoiniferter polymerization reactions resulting in a more time-efficient photoiniferter polymerization process.